Data Interpretation

When analyzing these data fields from a high torque screwdriving application with the payload properly defined, we should see that the actual currents start to deviate from target currents (in the main joints counteracting the tightening torque) during run down of the screw, with the largest deviation at the conclusion of the tightening as the torque reaction occurs.

If any of the actual current values deviating from its target flattens out to horizontal as if they have hit a limit, this could be due to joint torque maximum values or safety system limitations (restricting allowed joint torques). These safety limit values change significantly depending on whether the robot is steady or not. When the joint current does hit a limit, position deviation is expected to occur, i.e., actual joint position deviates from target position since the joint does not have any more torque available to maintain its position. Once the joint angles start to deviate, a protective stop will be triggered if any joint exceeds its allowed position deviation. This permitted deviation is also larger when the robot is in steady mode.

While a gap between the actual and target currents during the screw or nut tightening process is acceptable, hitting any of the joint torque limits should be avoided. Any visible deviation of joint positions is not acceptable, even if not triggering a protective stop. Similarly, protective stops and any other types of errors or warnings (even if occurrence is isolated and random) should never be accepted as part of a well-deployed application.